Tap for cutting internal screw threads



Nov. 30, 1943. P. CONTALDI TAP FOR CUTTING INTERNAL SCREW-THREADS Filed Jan. 15, 1945 =ill.

Patented Nov. 30, 1943 UNITED STATES PATENT OFFICE TAP FOR CUTTING INTERNAL SCREW THREADS Pasquale Contaldi, Philadelphia Pa.

Application January 15, 1943, Serial No. 472,461

Claims. (01. -141) The present invention relates to taps or tools for cutting internal screw-threads.

An object of the present invention is to provide a new and useful tap for cutting internal screwdescription, appended claims and accompanying drawing.

For the purpose of illustrating the invention, there are shown in the accompanying drawing forms thereof which are at present preferred, since the same have been found in practice to give satisfactory and reliable results, although it is to be understood that the various instrumentalities of which the invention consists can be variously arranged and organized and that the invention is not limited to the precise arrangements and organizations of the instrumentalities as herein shown and described.

Referring to the accompanying drawing in which like reference characters indicate like parts throughout:

Figure 1 represents a view partly in elevation and partly in cross-section of a tapered tap forming one embodiment of the present invention as it appears during the start of the screw-threading of a blank hole.

Figure 2 represents a view partly in elevation and partly in cross-section of a bottoming tap forming another embodiment of the present invention as it appears near the end of the screwthreading of the blank hole of Figure 1.

Figure 3 represents a plan view of the tap of Figure 1.

Figure 4 represents a vertical cross-sectional view of the tapered tap of Figure 1.

Figure 5 represents a plan view of the tap of Figure'2.

Figure 6 represents a vertical cross-sectional view of the tap of Figure 2.

Figure 7 represents a horizontal cross-sectional view generally along the lines 1-! of Figures 1 and 2.

Figure 8 represents a view generally similar to that of Figure 1 but illustrating how the tapered tap may be removed if broken off during the screw-threading operation.

Figure 9 represents a view generally similar to that of Figure 2 but illustrating how the bottoming tap may be removed if broken off during the screw-threading opertion.

In the embodiment shown in Figures 1, 3 and 4, I may provide a tap indicated generally by the reference character 20.

The tap 20, which is of hardened steel for cutting internal screw-threads in metal, is of the type known as a tapered tap for cutting an internal screw-thread, in a hole formed in metal or the like and includes a shank 2| and a square uppermost end 22 which is adapted to be gripped by a wrench or the like for rotation of the tap I during a threading operation.

A plurality (as for example 4) of grooves or flutings 23 extend generally axially of the tap 20 below the shank 2| thereof and divide the frusto-conical external screw-thread formed upon the lower end of the tap 20 into a corresponding number of axially-tapered screw-cutting segments 24. The grooves of flutings 23 may be circular or elliptical in cross-section as shown in Figures 3 and 7 and form a corresponding number of sharp cutting edges along the leading edges of the segments 24. The grooves or fiutings 23 serve to contain the shavings or cuttings produced during the screw-threading operation.

A hole 25' may extend along the axis of the tap 20 from the bottom or cutting end thereof to the upper end of the segments 24 as shown particularly in Figure 4. The hole 25 preferably has a polygonal cross-section. Where four grooves or flutings 23 are employed as shown in Figures 1, 3 and 7, the hole 25 has a square crosssection; the corners of the square being disposed adjacent the screw-cutting segments 24 and the sides of the square being adjacent the bottoms of the grooves or flutings 23 so as to cause a min imum weakening of the tap.

It is to be understood that, instead of the four grooves or flutings shown in Figures 1, 3 and '1, it is possible to employ three or five or more flutings to form three or five or more cutting segments, whereupon the inner hole will have a triagonal or pentagonal or other correspondingly polygonal cross-section, with the corners of the hole being adjacent the cutting segments.

Another embodiment of-the present invention is that shown in Figures 2, 5 and 6 in which a cylindrical bottoming tap 26 includes a generally cylindrical shank 21 and an upper square end 28 and a lower cylindrical external screw-thread which is divided into four screw-cutting segments 29 by four axially-extending grooves or flutings 30 (as shown in Figures 2 and 5) which are of circular or elliptical cross-section (as shown in Figures 5 and 7) A hole 3| may be formed along the axis of the tap 23 and may extend the entire length thereof, passing through the shank 21 and the square end 23 as well as intermediate the cutting segments 29 as shown particularly in Figure 6. The hole 3| may be of the same form as the hole 23 described hereinabove. Where the tap has four cutting segments as shown in Figures 2, 5, 6 and 7, the hole 3| is formed with a square cross-section; the comers 01' the square being adjacent the cutting segments 23 and the sides of-the square being adjacent the bottoms of the grooves or flutings 30 so as to produce a minimum weakening or the metal of the tap.

Taps of the character shown in Figures 1 and 2 are employed for cutting internal screw-threads in a hole formed in metal or the like, as for example the cylindrical blank hole 32 shown in Figures 1, 2, 8 and 9.

The tapered tap 20 of Figure 1 may be used to start the screw-threading operation: the upper square end 22 of the tap 20. being gripped by a wrench or other suitable tool to rotate the tap about its axis and, under simultaneous ax al pressure, to force the smaller end of the tapered cutting surface helically into the upper end of the cyl ndrical blank ole 32. The four leading'edees of the tapered cutting segments 23 form internal screw-threads upon the wall of the hole 32; the diameter Of the hole 32 being approximately the same as the diameter of the smaller end of the cutting surface of the tap 20. As the tap 2|! moves helically into the hole 32. the progressively larger diameter of the screw-cutt ng portion thereof gradually increases the depth of the internal screw-threads formed on the wall oi the hole 32.

After having been used to cut the t pered internal screw-threads w thin the hole 32. the ta pered tap 20 is backed out (by opposite rotation of the wrench) and the cylindrical bottoming tap 2G is inserted within the hole 32 and similarly rotated by a wrench or t e like to move it heli cally within the hole. The cylindrical cutting surface of the bottoming tap 26 has generally the same outside diameter as the maximum outside diameter of the cutting surface of the tapered tap 20. Thus, as the bottoming tap 26 moves helicallv within the hole 32, it produces a full cylindrical internal screw-thread extending generally the full axial length of said hole 32. Thereafter, the bottoming tap 28 is backed out (by op osite rotation of the wrench).

However, it happens quite frequently that taps (including tapered taps, bottoming taps and plugtaps) break of! during the screw-threading operation due to the very high torsional stress which must be applied to the square end thereof in order to form the screw-threads, especially in hard the inner portion of the tap to rotate it and back it out. As a result, a mechanic or machinist is frequently delayed forhours during the progress or important work until he is able to remove the broken off portion of the conventional tap by some make-shift means. It can readily be seen that the time and money lost by the breaking oil. of conventional taps is very considerable and represents a great waste of manufacturing facilities.

I have solved this problem b providing an inner axial opening in the tap, the opening being of polygonal cross-section corresponding to the number of cutting segments or the tap; the corners of the polygon being disposed adjacent the cutting segments to produce a minimum reduction in the metal of the tap and to produce a minimum weakening in the tap. With the corners of the polygon so disposed, the minimum thickness of the wall of the tap (wihch is adjacent the bottoms of the grooves or flutings thereof) is adequate to withstand the torsional stress normally applied during the screw-threading operation.

The inner polygonal hole may extend either all the way through the tap (as in the embodiment of Figures 2, 5 and 6) or it may extend only partly through the tap, as for example through the screw threaded portion thereof (as in the embodiment of Figures 1, 3 and 4) Where the tap breaks off at a point below the surface of the hole 32, as shown for example in Figure 8, the inner polygonal axial hole or openf ing thereof is exposed and the broken-off inner portion thereof can be backed out of the hole by employing any suitable tool 33 having a male portion of correspondingly polygonal cross-section which is inserted within the axial opening; the tool then being rotated to turn the brokenoff inner portion of the tap and to back it out of the hole.

Where the tap of Figures 2, 5 and 6 is broken 0!! at or slightly above the surface of the hole 32 as shown in Figure 9 so that the exposed surface of the tap is insufllcient to permit it to be gripped by the conventional wrench, the inner axial polygonal hole or opening 3| thereof is exposed and, again, the tool may be inserted within said hole 3| and rotated to back the tap out of the hole 32.

It is apparent that, by employing the novel tap of the present invention, the excessive time and labor consumed in removing broken conventional taps is entirely eliminated and that a broken tap may be removed from the work in a matter of a few seconds. At the same time, this elimination of lost time and labor is accomplished without any conisderable weakening in the tap.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being had to the appended claims rather than to the foregoing description to indicate the scope of the invention.

' Having thus described my'invention, what I claim as new and desire to protect by Letters Patent is:

1. A tap for cutting internal screw-threads in a hole, comprising an elongated member of hardened metal or the like having external gripping surfaces at one end thereof and having the other end thereof externally helically corrugated, a plurality of longitudinally extending grooves formed upon the corrugated end of said member and forming .a plurality of circumferentially spaced generally longitudinally extending externally helically corrugated thread-cutting segments thereupon, said elongated member having a generally central longitudinally extending opening formed in the corrugated end thereof, said opening having a polygonal cross-section with the comers oi the polygon disposed adjacent said thread-cutting segments and with the sides of the polygon disposed adjacent said grooves, said opening being exposed upon breakage of the corrugated end of said member to permit insertion of a complementary male rotating tool within said polygonal opening for backing the broken-01f end of said tap out of said hole.

2. A tap for cutting internal screw-threads comprising an elongated member of hardened metal .or the like having a plurality of circumferentially spaced generally longitudinally extending externally helically corrugated cutting segments at one end thereof and having a shank provided with external gripping surfaces at the ,other end thereof, said elongated member having a generally central longitudinally extending opening formed therein, said opening having a polygonal cross-section with the corners of the polygon disposed adjacent said cutting segments.

3. A tap for cutting internal screw-threads comprising an elongated member of hardened metal or the like having a plurality of circumferentially spaced generally longitudinally extending externally helically corrugated cutting segments at one end thereof and having a shank provided with external gripping surfaces at the other end thereof, said elongated member having a generally central longitudinally extending opening formed therein, said opening having a polygonal cross-section with the corners of the polygon disposed adjacent said cutting segments, said opening and said cutting segments being of generally the same longitudinal dimension.

4. A tap for cutting internal screw-threads comprising an elongated member of hardened metal or the like having a plurality of circumferentially spaced generally longitudinally extending externally helically corrugated cutting segments at one end thereof and having a shank provided with external gripping surfaces at the other end thereof, said elongated member having a generally central longitudinally extending opening formed therein, said opening having a I polygonal cross-section with the corners of the polygon disposed adjacent said cutting segments, said opening extending generally the full longitudinal dimension of said elongated member.

5. A tap for cutting internal screw-threads comprising an elongated member of hardened metal or the like having a plurality of circumferentially spaced generally longitudinally extending externally helically corrugated cutting segments at one end thereof and having a shank provided with external gripping surfaces at the other end thereof, said elongated member having a generally central longitudinally extending opening formed therein, said opening having a generally non-circular cross-section, said openin'g being largest in transverse dimension ad- 30 Jacent said cutting segments.

PASQUALE CONTALDI. 

